1. Field of the Invention
This invention relates to a method and apparatus for comminuting solid particles and more particularly to such a method and apparatus in which the presence of acid gas and air toxics with the combustion of such particles is minimized.
2. Description of the Related Art
Micronized limestone is used in agriculture, industrial fillers, environmental controls and the construction trade. Micronized cement is useful in the building industry.
Micronized coal is used as an energy source in the generation of electricity while minimizing the NO.sub.x in the flue gases. Micronized limestone/dolomite or hydrated lime are used for SO.sub.2 cleanup in the flue gases of power plants. Micronized coal burns with a flame velocity similar to natural gas or fuel oil and with a short flame that allows the heat energy generated in the combustion to be readily transferred to the water walls of the boiler. This results in an increased boiler rating and less heat losses through the ducts and flue stack. Due to its large surface area, micronized coal is more volatile than pulverized coal. In staged combustion processes, micronized coal leads to a much deeper NO.sub.x control with low NO.sub.x burners in the main combustion zone of the boiler. In addition micronized coal provides a more complete carbon burnout in the fuel and therefore upon combustion yields a micronized fly ash with low carbon content which is of considerable value in the construction industry as a substitute for cement in high strength concrete formulations.
The combustion of micronized coal requires no excess air and results in minimized NO.sub.x in the flue gases. Micronized limestone, dolomite or hydrated lime are most valuable in the dry hot scrubbing of flue gases and afford effective aqueous scrubbing thereof as these particles have much larger reactive surfaces for the SO.sub.2 scrubbing. This results in a more complete utilization of the sorbents resulting in substantial savings in the flue gas clean up section of a power plant in conforming to the requirements of the Clean Air Act.
The use of micronized coal as the fuel for generating electric power in conjunction with micronized limestone/dolomite or hydrated lime for flue gas clean up thus has significant advantages over the use of conventional fuels such as fuel oil or pulverized coal and is much less expensive than natural gas.
In my U.S. Pat. No. 5,695,130 issued Dec. 9, 1997, a grinding system is described in which rotating screens with wide mesh openings are first used to comminute particulate material through spiral vortexes and such comminuted material is then fed to circular vortexes formed between rotating discs and stationary plates where the final grinding of the particulate material is accomplished and the final comminuted material is separated from the gas streams by centrifugal fans. In this patent, the control of SO.sub.2 and NO.sub.x in the main combustion zone of a boiler is described, this by co-firing micronized coal and micronized limestone, the active scrubbing agent being CaC.sub.2 which is formed in situ in the flames.
In my co-pending application Ser. No. 09/339476 now abandoned of which the present invention is a continuation in part, a micronization process and apparatus is described which employs an externally pressurized fluid bed where coarse grinding is accomplished with centrifugal air vortexes generated by fast rotating rotors. The coarser particles are recycled from the pressurized stream by fast rotating screens for further comminution; the passing finer particles are driven through a vertical stack of fast rotating screens which effect the superfine and ultra fine grinding of such particles without external classifiers and recycling. The system and method of the present invention applies the basic technology of my prior patent application in implementing the micronization of solid particles such as coal, limestone/dolomite, hydrated lime, and other sorbents by passing the modified micronized particles through a vertical stack of fast rotating screens wherein homogenization and further micronization take place and compounding of the products occurs with additional sorbents and catalysts while fresh surfaces are being created. A significant improvement is provided in the present invention by the surface modification of such particles with the injection of specific chemicals and sorbents onto the particles to scrub their surfaces which results in a substantial lowering of the NO.sub.x and SO.sub.2 in the emissions from the combustion residue.
In a US Department of Energy publication entitled Clean Coal Technology Demonstration Program, published in March, 1999, the commercial scrubbing of NO.sub.x and SO.sub.2 is described. Various approaches are described. In one of these approaches SO.sub.2 is removed by employing aqueous slurries of hydrated lime ("Gas Suspensions Absorption," "Confined Zone Suspension") or the aqueous suspension of limestone("Advance Flue Gas Desulfurization," "Confined Zone Suspension"). Dry injection of limestone may be done in the upper reaches of the boiler and an "activator reactor" may be employed where water is injected to complete the scrubbing action("LIFAC Injection Desulfurization"). This dry injection approach is claimed to remove about 70% of the SO.sub.2 as compared with 95% removal in the present invention.
Another approach described to lower NO.sub.x emissions employs "Low-NO.sub.x " burners providing a staged combustion of the fuel with an oxygen deficient medium in the main combustion zone and completes the process with "overfire air" by creating an oxygen rich medium above the reburn zone, in the burnout zone of the boiler. This process is claimed to reduce the NO.sub.x by about 68% as compared with 90% removal in the present invention. Still another approach described is "micronized coal reburn" wherein micronized coal and recycled flue gas are injected into the reburn port of the boiler, i.e. above the main combustion zone and below the burnout zone of the boiler. This is claimed to yield up to a 56% removal of NO.sub.x as compared with up to 90% removal in the present invention:
A reduction in NO.sub.x comparable to that in the present invention can be obtained by a "Selective Catalytic Reduction" but this approach is very costly(estimated at more than three times the cost involved in the use of the present invention). Also, processing difficulties have been experienced with this approach in that the heavy metal catalysts present in the form of ceramics and zeolites are mechanically and thermally fragile as to long term exposure in the boiler medium, and the dust inherent in the flue gases is deposited on these catalysts and requires removal with soot blowers or sonic horns.
Still another approach described in this report for the simultaneous removal of SO.sub.2 and NO.sub.x from the residue is known as the "Milliken Clean Coal Demonstration Project." In one system described, urea injection in the boiler for NO.sub.x reduction of 42% and wet scrubbing with hydrated lime to provide an SO.sub.2 reduction of up to 98% is utilized. In another system called the "NOXSO Flue Gas Cleanup," a high temperature fluidized bed absorber with porous alumina beads impregnated with molten sodium carbonate is employed. This system is claimed to provide a 75% NO.sub.x reduction and a 98% SO.sub.2 reduction. It is, however, quite expensive due to the required high temperature regeneration of the sorbent with natural gas.
Other approaches described in the report include the "LIMB Extension Coolside Flue Gas Cleanup" which uses low-NO.sub.x burners, the injection of limestone in the upper reaches of the boiler, above the "overfire air" and additional duct injection of hydrated lime and sodium hydroxide resulting in a NO.sub.x reduction of 70% and an SO.sub.2 reduction of 70%; the "Gas Reburn Sorbent Injection" using 20% of the fuel as natural gas injected in the reburn port followed by sequential duct injections of hydrated lime, sodium carbonate, and water which results in a NO.sub.x reduction of 75% and an SO.sub.2 reduction of 60%; the "Integrated Dry NO.sub.x /SO.sub.2 Emission Control System" which provides a NO.sub.x reduction of 80% and an SO.sub.2 reduction of 70%; the "SOX-NOX-ROX-BOX" process using duct injection of hydrated lime and sodium carbonate for SO.sub.2 control and duct injection of ammonia for NO.sub.x control associated with a "Hot Baghouse" containing assorted heavy metal catalysts which results in a NO.sub.x reduction of 90% and an SO.sub.2 reduction of 90%; and the "SNOX Flue Gas Cleaning" process wherein ammonia is injected in the flue gases with a "first catalytic converter" for NO.sub.x cleaning and a "second catalytic converter" for converting SO.sub.2 to SO.sub.3 (with the isolation of concentrated sulfuric acid as a co-product) which results in a reduction of NO.sub.x of 94% and an SO.sub.2 reduction of 95%.
Most of these prior art systems are incapable of producing the NO.sub.x and SO.sub.2 reduction of the present invention. Those which are capable of high level reduction such as "Selective Catalytic Reduction," "SNOX Flux Gas Cleaning," and "SOX-NOX-ROX-BOX" are much more expensive to implement than the system of the present invention. Further, they present problems in that the assorted heavy metal catalytic particles are degraded in the course of operations resulting in a costly replacement of the spent catalytic columns. Further, their disposal requires costly procedures for toxic waste of heavy metals and their compounds. Injection of gaseous ammonia into the flue gases creates additional risks as the presence of SO.sub.3 and moisture reacts with the ammonia to yield ammonium bisulfate, which tends to corrode the metallic ducts of the boiler.